PROJECT SUMMARY Mutations in RAD51 genes confer genome instability phenotypes associated with infertility, birth defects, and cancer. Each of the RAD51 family members are central components of the error-free double strand break (DSB) repair pathway known as homologous recombination (HR), and RAD51D was recently identified as an ovarian cancer susceptibility gene. Even though it is known that HR defects predispose individuals to malignancy and confers cancer cell sensitivity to DNA damaging chemotherapy agents, single-protein analyses have revealed little functional information. Therefore, we performed RAD51D protein interaction screens and identified RNF138, a novel RING domain E3 ubiquitin ligase enzyme. Ubiquitin is a 76-amino acid protein that modifies protein functions or marks them for proteasome degradation, leading to the central hypothesis of this application: RAD51D forms adaptive complexes that are tightly regulated by RNF138 to process DNA breaks and maintain genome integrity. To test this hypothesis, the first aim will determine how RNF138 and RAD51D interaction regulates the HR DNA damage response pathway. The second aim will identify the ubiquitin linkage arrangements and sites along the RAD51D protein and their association with DNA damage response. Over the course of this project, undergraduate, graduate, and Doctor of Pharmacy students will receive outstanding biomedical research training. Undergraduate students are recruited from the highly selective University of South Carolina Honors College, which is ranked first by the Public University Honors Program Review of Fifty Public University Honors Programs. Therefore, investigating RAD51D post-translational modifications will uncover mechanisms of carcinogenesis pathways and identify potential cancer drug targets as well as provide training in state-of-the-art biomedical science techniques.